Hybrid technologies may be grouped into “full hybrid” and “micro-hybrid technologies.” Full hybrid technologies may contain a hybrid propulsion system that utilizes at least two different sources of torque for propelling the vehicle. As one non-limiting example, a hybrid propulsion system may combine a conventional propulsion system that includes an internal combustion engine and a stepped-ratio change automatic transmission with an electric propulsion system that includes one or more electric motors and a rechargeable energy storage device, such as a battery, that can power the electric motors or store energy to improve fuel economy over the conventional vehicle. A hybrid electric vehicle (HEV) typically provides different powertrain operating modes with the engine running or shutdown depending on the vehicle operating conditions, battery conditions, and driver's propulsion request. Hence, one of the major functions that an HEV provides is the ability to start or stop the engine during certain conditions. When the engine is running, the electric portion of the propulsion system may be used to assist the engine in providing the required vehicle propulsion. During the conditions when the engine is shutdown, the driver's propulsion request can be provided entirely by the electric motor, independent of the internal combustion engine.
Micro-hybrid technologies may contain Integrated Starter Generator (ISG) machines that are not capable of propelling the vehicle but are capable of supporting a rapid start of the engine as well as on/off operation of the engine while the vehicle is stationary. Thus, using micro-hybrid technologies, motor vehicles can be designed to employ certain aspects of HEV technology to reduce fuel consumption, but without use of a hybrid drivetrain. In such vehicles, sometimes called micro-hybrids, shutting down the engine during conditions where the engine operates at idle speed is used to reduce fuel consumption in a conventional powertrain that includes an internal combustion engine and a stepped-ratio change automatic transmission, but no electric machine for driving the wheels.
In conventional ISG based micro-hybrids, the ISG may be placed in series with the internal combustion engine and transmission system. The ISG takes the place of the traditional engine starter as well as the traditional alternator/generator that generates electric power for accessories and battery storage. The ISG may allow the engine to go from zero rpm to several hundred rpm in a fraction of a second, thus providing seamless start-stop capability to the driver in addition to increased fuel economy and reduced emissions.
The primary condition that is checked by a micro-hybrid powertrain control system before stopping the engine is that the driver has applied the brakes and the vehicle is stopped since the engine would typically be idling during these conditions in a conventional vehicle. Once the driver releases the brake pedal indicating a request for vehicle propulsion, the powertrain control system will automatically restart the engine.